In many processes such as ore beneficiation and the manufacture of portland cement, powdery or finely divided material must be agglomerated into bodies of more or less uniform size and shape that are intended to be roasted in a kiln or subjected to some other treatment.
One type of apparatus heretofore used for the purpose was a compacting mill such as is disclosed in U.S. Pat. No. 3,396,952, wherein fine material was fed into the nip of a pair of opposed cylindrical rolls that rotated in opposite directions about parallel axes. Compacted by the rolls, the material emerged from between them in the form of a sheet that was passed more or less directly to a breaking device that broke the sheet into so-called flakes. Inasmuch as the sheet tended to break at random, the flakes thus obtained were irregular in both shape and size, and some of the fines were freed from the compacted mass. The material was therefore passed directly from the breaking device to a separating screen at which the fines and the undersized flakes were separated from the remainder of the compacted material. Usually the undersize material that fell through the screen was circulated back through the compacting mill to be recycled.
For many applications, especially where the agglomerated material was subjected to a heat treatment involving critical time and temperature relationships, the random sizing of the flakes that passed over the separating screen was a disadvantage. However, even where the lack of uniformity of useable flakes was not a serious disadvantage, fines and undersized flakes tended to be produced in undesirable quantities. Frequently as much as 50% of the material had to be recycled after screening, and of course such recycling caused a corresponding decrease in the rate of production of useable flakes.
Where bodies of substantially uniform size and shape were needed, it was usual to employ the slower, more expensive and more troublesome briquetting technique. Briquetting is accomplished by means of apparatus such as is disclosed, for example, in U.S. Pat. No. 3,713,763, wherein the material to be compacted is fed through the nip of a pair of rolls that are formed with mating concave pockets in their cylindrical surfaces. The formation of the individual pockets in such rolls is difficult, and the rolls must be in such angular relationship to one another, and must be so synchronized in their rotations, that at the nip of the rolls the pockets in each roll are always directly opposite mating pockets in the other roll.
Although balling pans and balling drums can be used for pelletizing certain materials, such devices usually require that moisture be added to the material for pelletization. For some purposes, therefore, pellets formed with balling equipment must be dried before they can be subjected to further processing, whereas the addition of moisture to material to be put through a compacting mill or a briquetting machine is often unnecessary. Furthermore, pelletizing apparatus merely effects agglomeration of the fines into small masses and does not subject them to compacting pressure. For some applications, the greater density that results from compacting, or some other physical property that compacting produces, may be necessary or desirable.
Thus there has been a long-standing need, heretofore unsatisfied, for compacting apparatus by which agglomeratable fine material can be converted into pellet-like or tablet-like bodies that are substantially uniform in size and shape; and it is the general object of the present invention to provide simple, inexpensive and efficient apparatus which satisfies that need.
Another object of this invention is to provide improvements in a basically conventional compacting mill whereby the mill is enabled to produce a compacted sheet which tends to break along predetermined planes that extend transversely to the thickness of the sheet and are in mutually perpendicular relation to one another, so that the bodies formed by breakage of the sheet are substantially uniform in size and shape.
A further object of the invention is to provide apparatus of the character just described by which a compacted sheet is produced that can be broken into substantially uniform tablet-like or pellet-like bodies with minimal production of fines as a result of the breakage, so that relatively little recycling need take place in the operation of apparatus embodying the invention.
It is a more specific object of this invention to provide improved compacting rolls for an otherwise conventional compacting mill, capable of being installed in existing compacting mills to modify them for achievement of the above-stated objectives, which compacting rolls can be readily made by generally conventional machining processes.
It is also a specific object of this invention to provide compacting apparatus that has lower cost, higher production speed and greater production efficiency than briquetting apparatus, requires no maintenance of a particular angular relationship between its rolls, and nevertheless produces tablet-like bodies that are nearly as uniform in size and shape as briquettes.